The Government has quietly cut the “landscape value” it assigns to some of
England’s finest countryside by up to 90 per cent.

The move, which has serious implications for the green belt, comes amid reports that ministers are pushing for the relaxation of planning laws to allow more building on greenfield sites.

The new values emerged through Freedom of Information Act requests by campaigners against the controversial HS2 high-speed rail line, which the Government wants to drive through London’s green belt and the Chilterns, an Area of Outstanding Natural Beauty (AONB).

“Landscape impact value” tries to quantify the cash value of green space to Britain and the environment. It is used to examine how much damage development will do and is different from the commercial or sale value of the land.

In an official 2010 assessment by HS2 Ltd, which is building the line, the 109-mile route from London to Birmingham was estimated to cause £4.3 billion of damage to the landscape.

On the London green belt and Chiltern AONB sections alone, the “landscape impact” was estimated at just under £1.1 billion.

A new assessment by the Department for Transport estimated that the impact damage for the route is £957 million, 78 per cent lower.

The impact of the line in its London green belt and Chilterns AONB sections was said to be £114 million, a tenth of the 2010 figure.

Some reductions are because of measures taken to reduce the line’s impact. There will be longer tunnels and more “green tunnels” — roofed-over cuttings — than in the original plans. But the most dramatic reductions have been achieved by redefining the most valuable areas of countryside, including the green belt and AONB, as worth far less than before.

The valuation system classifies open space into seven categories. The most valuable is urban parkland. AONBs and other unspoiled natural land are the second most valuable and green belt land is the third most valuable category.

In the new assessment, almost all the London green belt and AONB land along HS2’s proposed route has been demoted, for valuation purposes, to the lowest category of “intensive farmland,” worth just £103 per hectare.

Hilary Wharf, of the HS2 Action Alliance, warned the move “establishes a precedent that the most precious green space can be devalued whenever the Government wants to build on it badly enough”.

“We may see this happening with future big projects all over the country,” she said.

The landscape impact value figure has not been counted in HS2’s business case. But a committee of MPs this year recommended that it should be, which may account for the Department for Transport’s wish to reduce the figure.

According to reports, George Osborne, the Chancellor, wants to simplify rules that allow developers to use part of the green belt for building, as long as they give replacement green land in return.

He is being strongly resisted by ministers at the Department for Communities and Local Government — one of whom, Grant Shapps, said the issue was “settled.”

Research by the Campaign to Protect Rural England (CPRE) published last week warned that green belt land around cities and towns was under threat from plans for at least 81,000 homes, as well as roads, industrial parks, mines and airport growth.

The plans include the expansion of Birmingham airport, proposals for freight terminals near St Albans, Luton and St Helens, an opencast coal mine at Broxtowe, Nottinghamshire, and a hotel and golf course in Leatherhead, Surrey.

Government plans to reduce protection for other greenfield land were defeated last year after protests from countryside groups and residents.

The Department for Transport said the “large discrepancy” between the impact assessments of HS2 was because the earlier figure was “crude” and took no account of mitigation, such as extra tunnelling, and was “not directly comparable” to the more sophisticated exercise it had now carried out.